Optimizing Power Begins With Architecture

Wednesday 22nd November 2017

Every mobile device design team focuses on reducing power consumption, yet few designers realize those same practices can increase efficiencies 50 percent or more in stationary applications.By: Mark Andrews

Even if not having to grapple with a finite mobile device powersupply, design teams often overlook potential market advantages by not making lowpower consumption a priority at the initial architecture stages of a newproject. Those who do can increase efficiency 50 percent and speedtime-to-market, according to the design automation experts at Cadence.

Dealing with power consumption as a key consideration at thearchitecture stage saves time and eases the burden on engineering teams thatwill inherit and grapple with a design, whether it was optimized at thearchitecture stage, or not. Too often, assumptions are made about what ispossible when it fact the team is proceeding on outdated concepts, according toCadence Sr. Principal Project Manager, David Pursley.

Cadence (UK and global), specializes in design automation tools thatseek to remove assumptions and preconditions from a design equation that haveno business being there. Cadence has found that when customers have a problemwith power management systems they often discover a common set of issues atwork that are impeding design team progress.

First is the assumption that methodologies which have been aroundfor years and are considered to be “givens" among marketplace leaders areaccurate. Many designers believe that ‘XYZ’ company got to be number one bygetting the details right when in fact there can be myriad reasons why onecompany’s products are successful and others are not. Even top-selling productscan have flaws; unnecessarily high power consumption is often near the top ofany list. But what happens when designers’ assumptions are incorrect? Delays.Miscues. Missed opportunities.

Common misconceptions in low-power design frequently include:

•While a designer’s intuition about a system’s potential performanceis often 80 percent correct, ‘intuition’ about power utilization is oftensubstantially incorrect. Cadence has found this can be off by as much as 50percent.

•Designers working on architecture often believe that next-stagework at the register-transfer level (RTL) will correct architecture deficienciesand that the remainder will be ‘fixed’ during the march towards implementation.In reality, approximately 80 percent of power optimization needs to beaccounted for at the architecture stage.

•Power utilization only counts in mobile applications. Wrong. Thepotential for creating greater efficiency is heavily dependent on use-case, andwhile this is especially true for mobile applications, it is neverthelessimportant in wired applications, especially in cases where the consuming publicwill choose between an energy efficient design with a standards body’s certificationand ‘Brand X’ that carries nothing to reassure potential purchasers that theproduct sips instead of guzzles energy.

•Peak-power is not necessarily thought of as a key consideration,but it is especially so in terms of long-term reliability. Average consumptionfigures alone are insufficient measures and cannot be used solely to predictperformance as a stand-alone approach.

These common misconceptions have led Cadence to conclude that low-poweroptimization is an objective that must be addressed at every point within thedesign flow – from architecture all the way to GDS, with architecture being themost important (and relatively easiest) stage at which to gets things rightsince up to 80 percent of a device’s power consumption profile will be determinedat this stage.

Why do some of the world’s best companies have design teams thatwork under assumptions about power utilization and consumption that today’sbest research has proven to be flawed? Differences abound, but one of theclassics finds designers reassuring themselves that a certain approach is safeto pursue because, ‘we have always done it that way,’ or the even more popular,‘this isn’t a mobile app, so why should we care about power?’ Cadence has alsofound a rather surprising trend in some established design outfits: they relyon an industry standby to save themselves from themselves: the ‘Hail Mary’approach—asking a ‘higher power’ for design assistance once a new projectleaves the hands of one team and heads to another is not strategy, it isdesperation.

Cadence has found time and again that unless a design team is onlyattempting a small tweak to an existing, well-characterized design that will beused by the same customer in the same applications as it was originally, onemight be able to get away with a corner-cutting approach to optimizing powerutilization. Otherwise the designer needs to start from a solid architecturalbasis.

For low-power stationary applications the team does not have to getanywhere near as millivolt-conscious as those working on mobile design projects,but HPC/cloud applications, cost-sensitive applications without active coolingand high-reliability systems now also have tight and unforgiving power budgets.And then there is the IoT. To realize the potential of hyper-connected sensorsand actuators across a cityscape in an internet of things (IoT) scenario, powerconsumption, self-powered systems and energy harvesting schemes are evidence ofjust how serious non-mobile applications have become about power utilization,energy storage and internetworking.

For IP power characterization, start with RTL or gate-level models.For a new IP, the designer might consider starting with high-level design (e.g.SystemC). Developing at high-level allows for quick turn-around architectureexploration that will enable the team to optimize for power. Many functions arebeing developed this way today including image processing systems, logicprocessors and controllers. If this isn’t an option, stick with RTL modelssince power estimation will be accurate; this approach should get the designwithin about 15 percent of the specified power consumption budget. For some,the idea of 15 percent (plus or minus) is way too great an error margin. But itshould be noted that +/- 15 percent is significantly better than the 50 percentright/wrong that usually results when designers simply follow their intuition.

Next, consider what the RTL team can be expected to achieve, and itwon’t be cutting power consumption by 50 percent. The RTL group canrealistically be expected to improve on an architecture team’s work by anywherefrom 15 percent to as much as 30 percent. But remember that the better power isoptimized at the architecture stage, the less RTL designers have to worry about,which automatically ‘improves’ the architecture work in their minds before theyeven begin.

Squeeze everything that can be had out of power before the design ishanded over to implementation. Make certain that the design is within ~5-10percent of budget before there is any handoff. Implementation can fine tune adesign but they cannot save anyone’s reputation if the power utilization issubstantially off target. What they need to focus on—and what they expect towork towards—is squeezing out the last few percentage points needed to give salesand marketing a solid product that can win against the competition.

If architecture has done its job then implementation can focus onthe last few percent of power gains, signal integrity, and making sure that thermalparameters are respected without any budget-busting; that the project will notbe suddenly burdened with an active cooling system not in the original specs,or that thermal issues are suddenly pushing the red line. Thermal issues,especially peak thermal spikes, can affect EM as well as timing and couldresult in a thermal runaway. Even the largest, most experienced companies canget this wrong, and with disastrous results—ask Samsung about their experienceswith thermal runaway in Lithium-ion batteries caused by a power managementsystem that didn’t get all the details quite right at one or another designstage.

Conclusion

Power optimization is not a back-end feature or tool. It’s a whole toolset and the responsibility of every engineering team all the way fromarchitecture to implementation; the bulk of that responsibility begins at the architecturestage. The right way to handle this is through an integrated flow of the typeenabled by tools provided by Cadence and similar companies that build theirreputations on speeding design flow while adding accuracy and optimization. Whyis integration important? Because a lot of getting the power budget undercontrol and within spec has to do with consistency in handling constraints andestimation throughout every stage of the design flow, which is pretty hard tomanage if a designer mixes her or his intuition with a mish-mash of tools.

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